Catalytic Cracking of Palm Oil: Effect of Catalyst Reuse and Reaction Time of the Quality of Biofuels-like Fractions

Abstract

This work systematically investigated the influence of catalyst reuse and reaction time on the yield and quality of organic liquid products (OLP) obtained in a cracking pilot plant at 450 °C and 1.0 atm. The distillation of OLP produced 04 (four) distilled fractions (gasoline, kerosene, and green diesel). The biofuels-like fractions are liquid mixtures with high content of hydrocarbons (alkanes, alkenes, and aromatics) with potential application as substitutes for fossil fuels in internal combustion motors. The quality of the biofuels was certified by physical-chemical analysis and FT-IR and GC-MS analysis. The experimental results showed the feasibility of applying the spent sodium carbonate twice in the catalytic cracking of vegetable oils. The physical-chemical properties (density, viscosity, acid value, saponification value, and flash point) of OLP decrease as the reaction time increases. The distillation of OLP yields 62.35% (wt.), producing green-like gasoline, kerosene, and diesel fractions rich in hydrocarbons. Therefore, biofuel-like fractions produced by distillation of OLP have a great potential for replacing partially petroleum-derived fuels.

1. Introduction

The increasing global population has led to higher demand for energy to maintain or enhance living standards. At present, fossil fuels, such as coal, natural gas, and oil, remain the primary sources of energy serving modern society’s needs [1]. Notably, around 19% of the world’s energy demand is attributed to the transportation sector, which is experiencing rapid growth in energy consumption [2,3]. However, the limited reserves of fossil fuels and the adverse environmental impact of their combustion, contributing to global warming through CO₂ emissions, render their usage unsustainable in the long run [1]. Consequently, there is a pressing need for environmentally friendly alternatives, such as biofuels, to address the energy demands of the transportation sector sustainably and renewably [1,4].

Production of biofuels consisting of hydrocarbons can be carried out by several different routes [5]. Pyrolysis, also known as thermal cracking [6,7,8,9,10] and thermal catalytic cracking (or simply, catalytic cracking) [11,12,13,14,15,16,17,18,19,20,21,22,23], is one of the routes that can be effectively applied to lipid-based feedstocks (vegetable oils and fats, i.e., raw materials such as oilseeds, lipid-rich algae, residues such as used cooking oil or animal fat, among others) to produce hydrocarbons [24]. Different systems and reaction conditions have been applied to the thermal catalytic cracking of oleaginous feedstocks. However, it has usually been performed in a fixed-bed reactor, middle temperature, atmospheric pressure and in the presence of acid catalysts [11,12,21]. The major groups of catalysts applied to thermal catalytic cracking of lipid-based feedstocks are microporous (zeolite) [25,26], mesoporous (MCM-41 and SBA-15) [22,27], macroporous (silica-alumina, alumina) [28], and composite microporous–mesoporous catalysts [29,30], and aluminum-containing mesostructured materials (Al-MCM-41, Al-SBA-15) [11]. Thermal catalytic cracking of lipids such as vegetable oils can generate the following products: gaseous biofuels, including bio-hydrocarbons C₁-C₅, CO and CO₂; liquid biofuels, which are called bio-oil or organic liquid product (OLP); water and coke [5,31,32].

Numerous publications have explored the thermal catalytic cracking process of lipid-based feedstocks, examining various parameters’ influence, including temperature, reaction time, heating rate, gas flow rate, feed rate, particle size, presence of water, catalysts, and biomass composition [6,9,17,21,22,24,33,34,35]. These parameters significantly impact the yield and physical-chemical properties of the organic liquid product (OLP) [31], which are directly related to its chemical composition. The source of biomass and the operational conditions utilized during production play a crucial role in determining the OLP’s chemical composition production [9,36]. Despite extensive research in this field, limited attention has been given to the effect of reaction time on the quality of organic liquid products. A review by Guedes et al. [36] revealed that only nine articles investigated the influence of reaction time on OLP quality.

Typically, thermal catalytic cracking of lipid-based feedstocks using zeolite acid catalysts results in increased organic liquid product (OLP) yield, mainly comprising highly aromatic hydrocarbons in the gasoline range. On the other hand, amorphous catalysts tend to produce a higher amount of gases [37]. However, a drawback of zeolite-type acid catalysts is the production of OLP with a high content of carboxylic acids [38,39,40]. To address this issue, basic catalysts like sodium carbonate (Na₂CO₃) have been explored as an alternative for converting lipid-based feedstocks into biofuels with reduced carboxylic acid content through thermal catalytic cracking [13,23,33,41,42]. Junming et al. [41] investigated the cracking of soybean oil using basic catalysts (Na₂CO₃, Al₂O₃, K₂CO₃) at temperatures ranging from 623 to 673 K. The results showed that basic catalysts yielded biofuels with a chemical composition similar to diesel, featuring low acid values and favorable cold flow properties. Similarly, Weber et al. [42] studied the thermal degradation of free fatty acids and animal fat using Na₂CO₃ and 5% (wt.) water at 430 ± 20 °C in a pilot plant setting. The free fatty acids produced OLP with yields between 64% to 74% (wt.) and acid values ranging from 0.64 to 0.80 mg KOH/g. For animal fat, the OLP yield was between 60% to 70% (wt.), with acid values ranging from 0.5 to 1.8 mg KOH/g. The gaseous product yield ranged from 25% to 30% (wt.), and coke yields were between 4% to 6% (wt.). These findings suggest that basic catalysts offer a promising approach to producing biofuels with reduced acidity and improved properties.

Another important issue regarding the catalyst is to evaluate its recovery and reuse, as it is a way to make the process sustainable. In the literature, few studies have addressed catalyst recovery and reuse or investigated catalyst activity after repeated use [43]. However, the few studies investigating this parameter reported promising results, with some achieving good catalytic activity after catalyst reuse, even without the regeneration step. Furthermore, the effect of catalyst reuse without regeneration on OLP yield and physical-chemical properties has not been systematically studied [44,45].

The organic liquid product (OLP) resulting from the process consists of a mixture of oxygenated compounds and bio-hydrocarbons, which fall within the boiling range of gasoline, aviation kerosene, and diesel [5,20,31]. However, certain components, such as oxygenated compounds, can introduce undesirable characteristics to the OLP, limiting its direct use as a substitute for liquid transport fuel [46,47,48]. To address this issue, various types of distillation techniques, including atmospheric pressure distillation, have been employed for the fractionation and upgrading of the OLP [9,14,19,23,42,47,49,50,51,52,53,54,55,56,57,58,59,60]. These processes aim to provide biofuels with improved physical-chemical properties, either closer to the desired specifications or within the limits set by regulatory agencies in different countries [52]. The complex composition of the OLP results in a wide boiling temperature range, spanning from below 100 °C to around 250 to 280 °C [36]. Atmospheric distillation has proven advantageous due to its simplicity and maturity as a separation process commonly used in the petrochemical industry. It offers a technically feasible and economically viable solution for separating the diverse components in the OLP, ultimately generating biofuels with properties closely resembling those of gasoline, aviation kerosene, and diesel [5,59,60,61].

In their study, Capunitan and Capareda [47] investigated the fractional distillation of bio-oil under both atmospheric conditions and reduced pressure (vacuum). The results demonstrated that the process yielded high amounts of heavy fractions and led to a significant reduction in humidity and acid value. Chemical composition analysis revealed that aromatic and oxygenated compounds were distributed in the light and medium fractions, comprising approximately 15–20% of the total, while phenolic compounds were concentrated in the heavy fraction, accounting for approximately 53% of the total. The distillation process effectively separated the components, resulting in a heavy fraction with improved properties and composition. This heavy fraction can be further used as feedstock for future upgrading procedures or blended with other liquid fuels. Ferreira et al. [53] also studied the de-acidification of OLP through fractional distillation, employing laboratory-scale experiments with and without reflux and pilot-scale distillation using columns of different heights. The biofuels (distillates) obtained from the laboratory-scale experiments with and without reflux showed yields ranging from 62.15% to 76.41% (wt.) and 71.65% to 89.44% (wt.), respectively. On the pilot scale, the yield was 32.68% (wt.). For the pilot scale distillation, the acid values of the gasoline, kerosene, and light diesel fractions were 0.33, 0.42, and 0.34 mg KOH/g, respectively. GC–MS analysis of the OLP indicated that it consisted of approximately 92.84% (area) of bio-hydrocarbons and 7.16% (area) of oxygenates. Notably, the light diesel fraction contained 100% bio-hydrocarbons with an acid value of 0.34 mg KOH/g, demonstrating the effectiveness of OLP deacidification through fractional distillation.

In this context, the main objective of the present study is to investigate the thermal catalytic cracking process of palm oil on a pilot scale, aiming to obtain biofuels similar to gasoline, aviation kerosene, and diesel fuel. The specific objectives are as follows: (a) investigate the influence of the reaction time on the quality (physical-chemical properties) of OLP; (b) investigate the influence of spent catalyst reuse without regeneration on the yield and quality (physical-chemical properties) of OLP; (c) investigate the influence of the fractional distillation process of OLP on yield and quality (physical-chemical properties and chemical composition) of biofuels similar to petroleum-derived fuels such as gasoline, aviation kerosene, and diesel fuel.

2. Materials and Methods

2.1. Crude Palm Oil

Crude palm oil (Elaeis guineensis Jacq) was provided by Engefar Ltd. Ananindeua-Pará-Brazil). Crude palm oil has been physical-chemically characterized as described in the literature [23]. Palm oil has also been characterized by FTIR spectroscopy and GC-MS analysis. The absorption spectrum in biofuels’ infrared (IR) region was obtained with a Shimadzu FTIR spectrometer, model Prestige 21. The liquid samples were added between the KBr plates, using pipettes to allow the light pressure of the liquid and aiming to guarantee the uniformity of the formed film. The spectrum resolution was 16 cm⁻¹, and the scanning range was 400 to 4000 cm⁻¹. The fatty acid composition of palm oil was determined by gas chromatography according to the official method AOCS Ce 1-62, using a CP 3800 Variam auto-injector chromatograph equipped with Flame Ionization Detector (FID), as described by Llamas [62].

2.2. Catalyst

Fresh Sodium Carbonate (Na₂CO₃), commercial soda ASH Light (D50), with a purity of 98.0% (wt.) and spent Na₂CO₃ were the catalysts used in this work. Solvay Chemicals International SA (Brussels, Belgium) supplied sodium carbonate, which was characterized by FTIR spectroscopy, X-ray diffraction and thermal analysis (thermal gravimetric analysis and differential thermal analysis) as described in the literature [23]. The spent Na₂CO₃ was characterized only by X-ray diffraction, as described in the literature [23].

2.3. Experimental Apparatus

2.3.1. Thermal Catalytic Cracking Pilot Plant

The experiments were conducted in a semi-batch pilot pyrolysis plant at the Federal University of Pará (UFPA), composed of a 143 L atmospheric stainless-steel reactor, condenser, and separating drum. The apparatus was described in detail by Mota et al. [23].

2.3.2. Distillation Laboratory Unit

The OLP obtained was fractionated distilled in four fractions (gasoline range 1, range 2, aviation kerosene and diesel) in a laboratory scale glass unit consisting of an electrical heating mantle 315W (Quimis, São-Paulo-Brazil, Model: Q321A25/525), a 1 L round three neck bottom flask, Vigreux column of 3 stages, 700 mm length Liebig condenser coupled to a 230 mL separating funnel. A thermocouple (K-type) was connected to the round bottom flask to control temperature, and the other neck was used to draw samples.

2.4. Experimental Procedures

2.4.1. Thermal Catalytic Cracking

A total of 03 (three) catalytic cracking experiments were conducted in the pilot plant, 02 (two) in the 1° cycle with 10 and 20% (wt.) fresh Na₂CO₃ (FSC), and 01 (one) in the 2° cycle with 10% (wt.) reused Na₂CO₃ (RSC). All experiments were carried out at 450 °C and 1 atmosphere, using in a 1° cycle, 10 and 20% (wt.) fresh Na₂CO₃ (FSC), in order to investigate the influence of catalyst content on the yield and quality of OLP. Afterwards, a 3rd experiment (2° cycle) was carried out using the solid phase reaction product (M_Coke = M_Na2CO3 + M_Carbon) of an experiment carried out in the 1° cycle with 10% (wt.) fresh Na₂CO₃ as a catalyst, here named reused Sodium Carbonate (RSC), in order to investigate the effect of reusing the on the yield and quality of OLP. The reused Na₂CO₃ (RSC) was collected and submitted to the pre-treatments of disaggregation, drying at 105 °C, milling, and sieving. The sieved and classified particles were used as catalysts in the 2° cycle experiment with 10% (wt.) reused Na₂CO₃ (RSC).

2.4.2. Mass Balance of the Thermal Catalytic Cracking Process

Equation (1) was used to calculate the yield of the products (OLP, gas, and coke) of the cracking process:

$$YieldProducs=\frac{\mathrm{MProducts}}{\mathrm{MPalm} \mathrm{Oil}}\times 100$$

(1)

where m_{Palm oil} is the mass of crude palm oil (kg), m_Product is the mass of the product (kg). The mass of the gaseous products was estimated as the difference between the initial mass of oil and catalyst and the sum of the masses of organic liquid products and coke.

2.4.3. Evaluation of Reaction Time

During the thermal catalytic cracking experimental procedure (Section 2.4.1), we conducted a parallel study to investigate the impact of reaction time on the quality of OLP (organic liquid product). The approach involved collecting OLP samples (as shown in Figure 1) at various reaction times, specifically at 10, 20, 30, 40, 50, and 60 min, during the cracking of crude palm oil. To ensure proper preservation, these OLP samples were carefully stored in 1000 mL glass amber bottles. Subsequently, the OLP samples collected at different reaction times were combined to create a mixture, and this blended OLP was stored in a 50 L polyethylene container for further analysis and evaluation.

It is important to highlight that the experimental procedure to evaluate the influence of reaction time on OLP quality was adopted for the three conditions mentioned in Section 2.4.1.

2.4.4. Evaluation of Catalyst Reuse

To evaluate the influence of spent sodium carbonate reuse on the yield and quality of the OLP obtained, it was used for a consecutive cracking experiment without regeneration called reused sodium carbonate (RSC).

According to Section 2.4.1, an amount equivalent to 5 kg of catalyst (sodium carbonate) was used in the first cracking experiment. Therefore, the experimental procedure for the Evaluation of catalyst reuse initially consisted of removing the catalyst from the inside of the reactor. Then, the spent catalyst from the first cracking process was properly collected (Figure 2a) and impregnated with a cracking reaction by-product, coke, in the form of a heterogeneous mixture called the remaining mixture (Na₂CO₃ + coke). To significantly reduce the granulometry of the mixture, as shown in Figure 2b, it was subjected to a comminution process in a ball mill (CIMAQS.S.A.IND.ECOM, Series № 005), which had a diameter ranging from 1.55 to 3.97 cm. After comminution, the remaining mixture was dried in a preheated oven and kept at 150 °C for 30 min. After drying, the residual mixture, reused sodium carbonate, was removed from the stove and immediately weighed and packed in plastic bags with 500 g of RSC. The experiment was done in the same manner as for the fresh catalysts.

2.4.5. Distillation of the OLP

OLP (60 min reaction time) was subjected to fractional distillation and separated into the four fractions described based on the boiling point (B.P) ranges as follows: B.P < 90 °C gasoline range 1; B.P 90–160 °C gasoline range 2; B.P 160–245 aviation kerosene; 245–340 °C diesel. These temperature-based cuts were previously reported as producing bio-hydrocarbon fractions having similar properties as petroleum products [23].

The OLP was subjected to a gradual heating process to initiate the first distillation, targeting the first temperature range (B.P < 90 °C). The resulting distillate was collected in a separation funnel with a capacity of 250 mL and then stored in amber glass bottles for further analysis. Subsequently, the temperature controller of the heating mantle was adjusted to reach the second distillation range. The same procedure was carried out to obtain the two additional fractions. At the conclusion of the experimental procedure, we obtained four distilled fractions along with a bottom product, as depicted in Figure 3.

2.4.6. Mass Balance of the Distillation Process

Equation (2) was applied to find out the yield of the distilled fractions (green gasoline/Range 1, green gasoline/Range 2, green aviation kerosene, and green diesel) and bottoms products.

$$\mathrm{Yield} \mathrm{in} \mathrm{distilled} \mathrm{fractions} \left[\%\mathrm{wt}.\right]=\frac{m_{\mathit{DF}}}{m_{\mathit{OLP}}}\times 100$$

(2)

where $m_{\mathit{DF}}$ is the mass of distilled fraction (g), and $m_{\mathit{OLP}}$ is the mass of the organic liquid product (g).

2.5. Characterization of the OLP and Distilled Fractions

2.5.1. Physical-Chemical Properties

The OLP was characterized according to methods adapted from AOCS and ASTM standards. Acid Value was determined through AOCS Cd 3d-63 titration method. Briefly, 0.2 g of sample is dissolved into 50 mL of toluene/isopropanol combined solvent and titrated with standard 0.1 N isopropanol-KOH solution, using phenolphthalein as indicator. A blank solvent titration is also conducted to account for solvent contamination. The Acid Value (AV) is then calculated using Equation (3).

$$AV \left(\mathrm{mg}\frac{\mathrm{KOH}}{\mathrm{g}}\right)=\frac{0.1\times 56.1\times Fc\times \left(Va-Vb\right)}{M_{sample}}$$

(3)

where: Va—titrated volume of sample (mL)

Vb—titrated volume of blank (mL)

Fc—correction factor of KOH standard solution

M_sample—weighted sample mass (g)

Saponification Value was done according to AOCS Cd 3-25. Basically, 4–5 g of paper-filtered sample is combined with 50 mL of alcoholic KOH solution prepared by dissolving 40 g of low-carbonate potassium hydroxide in 1 L of KOH distilled ethyl alcohol, with cooling to 15 °C. In a twin setup with the blank (50 mL of alcoholic KOH solution), the sample is gently but steadily boiled under total reflux until the test portion is completely saponified (at least 30 min). After some cooling (excessive cooling must be avoided to prevent gel formation inside the flask), a phenolphthalein indicator is added, and the mixture is titrated with 0.5 M HCl until the pink color disappears. The saponification value (SV) is then determined by Equation (4).

$$SV\left(\mathrm{mg}\frac{\mathrm{KOH}}{\mathrm{g}}\right)=\frac{\left(B-S\right)\times M}{W}\times 56.1$$

(4)

where:

B—volume of 0.5 M HCl required to titrate the blank (mL)

S—volume of 0.5 M HCl required to titrate the sample (mL)

M—molarity of HCL solution

W—sample mass (g)

Specific gravity was analyzed according to an adaptation of ASTM D854, using a glass 5 mL pycnometer. The pycnometer is filled with samples and weighted. A previous calibration with water is done by filling the pycnometer with distilled water and weighing it. The specific gravity is calculated according to Equation (5).

$$SG \left(\frac{\mathrm{g}}{{\mathrm{cm}}^3}\right)=\frac{m_{sample}\left(\mathrm{g}\right)}{m_{water}\left(\mathrm{g}\right)}\times 0.997 \mathrm{g}/{\mathrm{cm}}^3$$

(5)

Refractive Index was measured according to the methodology of AOCS Cc 7-25 using an Abbé Refractometer and water calibration. Kinematic viscosity was measured according to ASTM D446 and ASTM D515 using a temperature-controlled water bath (Schot Geräte, Model: 520 23) and Cannon-Fenske capillary viscosimeter (n° 200). Sample flow time was measured three times, and the results presented are the average of those measurements. Flash Point and Corrosiveness to Copper were measured according to official methods ASTM D93 and ASTM D130, respectively.

Distilled fractions have been characterized with the same official methods adopted for the physical-chemical characterization of OLP plus the following official methods for Content of FFA (AOCS Ca 5a-40), carbon residue (ASTM D4530), and Ester value [63].

2.5.2. FTIR Spectroscopy

The FTIR analysis was conducted on Shimadzu Model Prestige 21 spectrometer. The scanning interval was 400–4000 cm⁻¹ with a scanning resolution of 16 cm⁻¹. The samples were prepared by adding a few drops of sample to KBr disks and analyzed.

2.5.3. Distillation Curve of the Distilled Fractions

Distillation curves were obtained by distilling samples according to ABNT/NBR 9619 method and automatic distillation apparatus (TANAKA, Tokyo-Japan, Model: AD6).

2.5.4. GC–MS Analysis of the Distilled Fractions

Chromatograms and mass spectra of distilled fractions were obtained through GC-MS analysis using a Shimadzu GCMS-QP2010 Plus, as described previously [23]. The percentage of present compounds is displayed in %area of the chromatogram, obtained by the ratio of the peak area of the compound to the total area of the chromatogram.

3. Results and Discussion

3.1. GC-MS Analysis of Crude Palm Oil

Lhamas [62] performed GC-MS analysis of the same crude palm oil used in the present study. They noted that it consists mainly of oleic acid (43.12%) followed by palmitic (36.62%), linoleic (12.93%) and stearic (4.92%), while lauric (0.46%), myristic (0.86%), palmitoleic (0.14%), linolenic (0.26%) and arachidic (0.35%) acids showed trace amounts. According to Lhamas [62], the result of the composition of crude palm oil agrees with the literature.

3.2. X-ray Diffraction

To investigate the influence of the reuse of sodium carbonate (Na₂CO₃) on the yield and quality of biofuels obtained from the thermal catalytic cracking reaction of crude palm oil at a pilot scale, an X-ray diffraction analysis was carried out for the sodium carbonate after each cracking reaction, and the results are shown in Figure 4. The results obtained for fresh sodium carbonate are discussed in detail by Mota et al. [23]. According to Figure 4, the predominant phases in the composition of fresh sodium carbonate are natrite and thermonatrite, corresponding to the respective file catalogs PDF 00-037-0451 and PDF 00-08-0448, as described in the literature [64]. For reused sodium carbonate after a cracking reaction, the compounds were identified according to the 2θ position for the highest intensity peaks: 100% (32.40°) with a spacing of 2.64; 67% (29.01°) with s spacing of 3.07 and 37% (36.74°) with a spacing of 2.44 according to the file catalogs PDF 00-029-1447, corresponding to the hydrated sodium hydrogen carbonate compound (Trona). This result shows the effect of the cracking reaction of crude palm oil on the surface and the structural and compositional reorganization of the catalyst. Besides, there was a change in the file catalogs of reused sodium carbonate after a cracking process compared to fresh sodium carbonate, resulting from the hydration of fresh sodium carbonate due to the production of water molecules during the secondary cracking reaction [64,65,66,67,68,69].

To understand the number of reuse cycles of the sodium carbonate, an analysis of the catalyst sample (sodium carbonate plus coke) from the second catalytic cracking reaction was carried out. In this sample, the compounds Natrite, Sodium Stearate, Sodium Phosphate, Dimethylglyoxime, and Potassium Acetate were identified, according to the file catalogs PDF 00-037-0451, PDF 00-002-0016, PDF 00-003-0394, PDF 00-003-0436, and PDF 00-001-0039. Except for the Natrite phase, the presence of these compounds indicates that they are products from numerous parallel reactions, which occur during the main reaction (cracking of triglycerides), where these compounds are deposited on the surface of the catalyst. The presence of such compounds may be related to the fact that crude palm oil was used, which did not undergo refining processes such as degumming and neutralization. Also, the diffractogram (C) of Figure 4 shows a range from 0 to 25° according to the 2θ position, which is characterized by the absence of crystallographic phases, indicating that there was a deposition of amorphous carbon-based compounds, which are present in the coke formed during the cracking process. This results in a decrease in the catalytic activity of sodium carbonate as it is reused without a previous regeneration step.

Therefore, the importance of the results obtained regarding the reuse of sodium carbonate without any regeneration step as a catalyst in the cracking process of vegetable oils is emphasized since, despite the reduction in its catalytic activity, it remains with the crystalline phase that characterizes it, which is the natrite phase. This indicates sodium carbonate can be reused more than once in the cracking process.

3.3. Influence of Catalyst Reuse on OLP Yield

In this study, the influence of catalyst reuse on OLP yield is assessed by comparing the yield obtained using the fresh catalyst and the yield obtained using the spent catalyst without regeneration for a consecutive cracking experiment, a reuse cycle. As shown in

Table 1. Process parameters and mass balance for the experiments of thermal catalytic cracking of crude palm oil using Fresh Sodium Carbonate (FSC) and Reused Sodium Carbonate (RSC) as the catalysts.

Process Parameters and Mass Balance	Catalyst [% wt.]
	10 (FSC)	10 (RSC)
Cracking Temperature [°C]	450	450
Mass of Feed [kg]	50	50
Mass of Catalyst [kg]	5	5
Process Time [min]	145	155
Initial cracking time [min]	60	75
Stirrer Speed [rpm]	150	150
Heating rate (°C)	n.r.	n.r.
Conversion [% wt.]
Mass of OLP (kg)	31.8	29.50
Mass of coke (kg)	4.00	1.86
Mass of gas (kg)	14.20	18.64
Mass of water (kg)	n.r.	n.r.
Yield of OLP (% wt.)	63.60	59.00
Yield of coke (% wt.)	8.00	3.72
Yield of gas (% wt.)	28.40	37.28
Yield of water (% wt.)	n.r.	n.r.

n.r., not rated.

, there was an increase in gas yield, to the detriment of a reduction in OLP and coke yield when 10% (w/w) of RSC was used, indicating that there was a reduction in catalytic performance in the first cycle of the spent catalyst without regeneration. The reduction in the catalytic performance of the catalyst can be explained by the impregnation of coke in the catalyst, as corroborated by the X-ray diffraction analysis (Figure 4), increasing the initial cracking time and process time of the experiment that used 10% (w/w) of reused sodium carbonate when compared to the same parameters of the experiment that used 10% (w/w) of fresh sodium carbonate. Karnjanakom et al. [70] compared the long-term stability of catalysts (2.5 wt% Cu/γ-Al₂O₃, 2.5 wt% Fe/γ-Al₂O₃, and 2.5 wt% Zn/γ-Al₂O₃) after regeneration and without regeneration by reusing them for up to 4 cycles and report that there was a significant reduction in catalytic performance in each spent catalyst cycle without regeneration due to deposition of coke in the catalyst, which can lead to coverage of active sites and pore blockage in the catalyst. Dandik and Aksoy [71] cracked residual sunflower oil in the different concentrations of fresh sodium carbonate, obtaining yields in OLP that varied from 19 to 48%.

Therefore, the results of OLP yield obtained in the present study from a cycle of reuse of sodium carbonate without regeneration are consistent with the literature. The 10% (w/w) RSC is a promising catalyst for the thermal catalytic cracking of palm oil, which can potentially be applied in a practical process and significantly reduce any costs associated with a step of regenerating sodium carbonate.

3.4. Influence of Catalyst Reuse on OLP Quality

3.4.1. Physical-Chemical Properties

The influence of catalyst reuse on the quality of the OLP is assessed by comparing the physical-chemical properties of the OLP obtained using the fresh catalyst and the physical-chemical properties of the OLP obtained using the spent catalyst without regeneration for a consecutive cracking experiment, that is, a reuse cycle. According to

Table 2. Physical-chemical properties of OLP produced with fresh and reused sodium carbonate.

Physical-Chemical Properties	Catalyst [% wt.]		ANP № 65 [73]
	10 (FSC)	10 (RSC)
Specific gravity at 20 °C [g/cm3]	0.950	0.980	0.82–0.85
Kinematic viscosity at 40 °C [mm2/s]	2.90	4.96	2.0–4.5
Corrosiveness to copper, 3 h at 50 °C	1A	1A	1A
Acid value [mg KOH/g]	8.98	39.00	n.r.
Saponification value [mg KOH/g]	9.19	89.46	n.r.
Ester value [mg KOH/g]	0.21	50.46	n.r.
Content of FFA [%]	4.51	19.62	n.r.
Refractive index	1.45	1.45	n.r.
Flash point [°C]	19.10	26.00	≥38
Carbon residue [%]	0.73	0.74	0.25

ANP: Brazilian National Petroleum Agency, Resolution № 65 (Specification of Diesel S10). n.r., not rated.

, the thermal catalytic cracking of crude palm oil with 10% (w/w) of fresh sodium carbonate produced an OLP with physical-chemical properties such as kinematic viscosity and corrosivity to copper within limits established by the resolution ANP № 65 for petroleum-derived diesel. In contrast, other properties, such as specific gravity and flash point, significantly differed from the limits established for petroleum diesel. When comparing the results of this experiment with data available in the literature, it can be emphasized that properties of the OLP, such as the acid value, which corresponds to 8.98 mg KOH/g, presented a satisfactory result since studies on thermal cracking and thermal catalytic of vegetable oils reached values much higher than those found in the present study. Seifi and Sadrameli [6] reached acid values of OLPs that varied between 66.16 and 123.41 mg KOH/g for experiments involving the thermal cracking of sunflower oil. Xu et al. [19] obtained an OLP with an acid value of 69.30 mg KOH/g for the thermal catalytic cracking of soybean oil using Na₂CO₃ as a catalyst. Prado and Antoniosi Filho [72] performed thermal and thermal catalytic cracking of soybean oil using bauxite as a catalyst and obtained OLPs with acid values ranging from 120–139 mg KOH/g and 44–91 mg KOH/g, respectively.

Table 2 also indicates that even after a cycle of reuse, spent sodium carbonate without regeneration continues to show catalytic activity for the thermal catalytic cracking reactions, despite showing that the thermal catalytic cracking with 10% (w/w) of spent sodium carbonate without regeneration produced an OLP with physical-chemical characteristics very different from those determined for the OLP obtained with fresh sodium carbonate. The value of the corrosivity property to copper is the only one within limits established by ANP № 65 for petroleum-derived diesel. Even so, the OLP obtained with 10% (w/w) RSC still manages to reach very satisfactory values of physical-chemical properties, mainly concerning its acid value, equal to 39 mg KOH/g, which is lower than the values shown in the literature, as explained above. These facts not only confirm the catalytic activity of the catalyst for the thermal catalytic cracking reactions but also raise the importance of the present study since most of the experiments carried out on the technological route for cracking vegetable oils were developed on a scale of bench or semi-pilot. Therefore, the reuse of spent sodium carbonate without regeneration was feasible when comparing the results presented in Table 2 with those obtained in thermal cracking and thermal catalytic reactions reported in the literature, even showing quite promising concerning some catalysts typically employed in the cracking process.

3.4.2. FTIR Spectroscopy

Both the thermal catalytic cracking experiment of crude palm oil using 10% (w/w) fresh sodium carbonate and the experiment that reused it without any regeneration step was successful in the cracking process since, in both experiments, the breakdown of triglyceride molecules into oxygenated compounds (carboxylic acids, fatty alcohols, and ketones) and, mainly, bio-hydrocarbons such as alkanes and alkenes was confirmed.

Figure 5 shows the results of the FTIR analysis of crude palm oil and OLPs produced from the thermal catalytic cracking of crude palm oil using 10% (w/w) fresh sodium carbonate and 10% (w/w) of reused sodium carbonate as catalysts, both at 450 °C, 1 atm and a pilot scale. The identification of the bands was performed according to a previous study [23]. The qualitative composition of crude palm oil determined by FTIR analysis was discussed in detail by Mota et al. [23] so that through the results obtained, it is possible to ensure that the spectrum of the analyzed oil is a typical spectrum of vegetable oils.

The comparison of the FTIR spectrum of crude palm oil with the OLP spectrum shows a displacement or disappearance of some bands, mainly those corresponding to the chemical functions of triglyceride esters, such as the band 1748 cm⁻¹ corresponding to the C=O bond, a group that appears in the range of 1750–1735 cm⁻¹ for simple aliphatic esters, corroborated by the bands 1259 cm⁻¹ referring to long-chain acid methyl esters, which appear close to the band 1250 cm⁻¹, in addition to the band 1033 cm⁻¹ for secondary alcohol esters [74,75]. The analysis of the FTIR spectrum of OLPs shows that they have very similar or similar bands, such as the bands 3425 cm⁻¹ and 3419 cm⁻¹ corresponding to axial deformation related to alcohols and phenols, characteristic of the O-H bond [14,23,51,53,74,75]. Also, the OLP spectrum shows characteristic bands of alkenes at 3078 cm⁻¹, in addition to saturated hydrocarbons, such as the characteristic bands of vibration of axial deformation of C-H, in the region of 3000 to 2840 cm⁻¹, corresponding to the function of hydrocarbons, more precisely of normal alkanes, and associated with these bands is the presence of the band 1375 cm⁻¹ referring to C-H angular deformation vibrations of methyl groups, which further corroborates the presence of saturated alkanes (normal paraffins) in the OLPs produced [14,23,51,53,74,75]. In parallel, both OLPs have peaks between 995 and 905 cm⁻¹, which are characteristic of an angular deformation outside the plane of the C-H bonds, indicating the presence of alkenes (olefins) [14,23,51,53,74,75]. It can also be highlighted the presence of bands between 720 and 660 cm⁻¹ with peaks characteristic of an angular deformation outside the plane of the C-H bonds in the methylene group (CH₂), indicating the presence of aromatic compounds [74,75].

According to Figure 5, the FTIR spectrum of the OLPs also shows that they reflect with the absorption of the identified bands, the results of the physical-chemical analysis, especially concerning acid value, since the OLP obtained with reused sodium carbonate showed a higher acid value than that obtained with fresh sodium carbonate, confirming the presence of carboxylic acids and ketones in these OLPs represented by the bands characteristic of this function, that is, bands of intense axial deformation, characteristic of the carbonyl groups (C=O), with peaks at 1707 and 1716 cm⁻¹ [14,23,51,53,74,75]. Notably, this functional group appears as a powerful band for the group C=O in the range of 1730–1700 cm⁻¹ for simple aliphatic carboxylic acids in the dimeric form. The presence of carboxylic acids is confirmed by the appearance of the O-H bond, which appears in the spectrum as a very wide band that extends from 3400 to 2400 cm⁻¹. This band has a characteristic peak close to or over 3000 cm⁻¹ [74,75].

3.5. Influence of Reaction Time on the Physical-Chemical Properties of the OLP

3.5.1. Physical-Chemical Properties

Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10 demonstrate the significant influence of reaction time on the physical-chemical properties of OLPs during the cracking of crude palm oil. As the reaction time increased from 0 to 60 min under the operational conditions used in this study, the values of physical-chemical properties sharply decreased for all three conditions employed. This observation can be attributed to the breakdown of palm oil molecules due to prolonged exposure to high temperatures and the catalytic effect, resulting in products with lower molecular weight than palm oil. A study by Zhao et al. [17] also investigated the catalytic cracking of carinata oil in a continuous flow fixed-bed reactor at 450 °C, with a reaction time of approximately 60 min. The findings from their study showed that carinata oil was converted into bio-hydrocarbons with molecules smaller than those present in crude carinata oil, leading to biofuels with lower density and viscosity. These results further support the notion that longer reaction times in the thermal catalytic cracking process result in the production of OLPs with improved physical-chemical properties compared to the original feedstock.

The data presented in Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10 demonstrate a noteworthy trend, indicating a substantial reduction in the physical-chemical properties of OLP between 10 and 30 min of cracking, followed by a relatively stable state beyond 30 min. This observation underscores the crucial role of reaction time in ensuring an efficient catalytic process [76]. However, exceeding the optimal reaction time can lead to a significant decomposition of the components present in the liquid products into lower molecular weight substances [77]. Moreover, it is important to consider that longer reaction times come with increased process costs [78]. Hence, it becomes essential to balance reaction time and the desired outcome of the thermal catalytic cracking process. By adhering to the optimal reaction time, one can achieve a higher yield of desired products and minimize the formation of undesired by-products. This approach is key to optimizing the production of OLP with favorable physical-chemical properties suitable for various applications, including biofuels and other value-added products.

Figure 7 shows that for thermal catalytic cracking experiments of crude palm oil using 10% (w/w) fresh sodium carbonate, 20% (w/w) fresh sodium carbonate and 10% (w/w) of reused sodium carbonate, the values of the kinematic viscosity of the OLPs reduce significantly in the first 10 and 20 min of the process. However, in this period, none of the three experiments managed to produce an OLP with kinematic viscosity values within limits established by ANP № 65 (Diesel S10) [73] or ASTM D975 (Diesel № 1-D S15) [79], which are 2–4.5 mm²/s and 1.3–2.4 mm²/s, respectively. After 30 min, the OLP kinematic viscosity values from the experiments that used 10% (w/w) fresh sodium carbonate and 10% (w/w) reused sodium carbonate continued to slightly decrease in such a way that after 40 min both experiments produced OLPs with values within limits established by ANP № 65 (Diesel S10) [73]. On the other hand, after 30 min, for the experiment that used 20% (w/w) of fresh sodium carbonate, the OLP kinematic viscosity values are little changed, so the kinematic viscosity values vary from 1.82 to 2.05 mm²/s in the period of 30 to 60 min of process. Therefore, the kinematic viscosity values of these OLPs are within limits established by ASTM D975 (Diesel № 1-D S15) [79].

The comparison between the experiments carried out with 10% (w/w) of fresh sodium carbonate and 10% (w/w) of reused sodium carbonate shows that the first promotes a more significant reduction in the kinematic viscosity values in the first 20 min than the second. This behavior can be explained due to the reduction in the catalytic activity of the reused sodium carbonate due to its impregnation with residual material (coke) on the catalyst surface, leading to the coverage of the active sites and the blocking of the catalyst pores, as reported by Karnjanakom et al. [70]. The result from the reduction in catalytic activity is a significant difference between the two experiments concerning the OLPs specific gravity values, as shown in Figure 6, and in the number of oxygenated compounds formed during the cracking reaction [65,66,67,68,69], as Figure 8, leading to higher values of kinematic viscosity, higher values of acid value and higher values of specific gravity in the first 20 min of the process. The higher the Na₂CO₃ content, the lower the oxygenate concentration in OLP, which is the dominant effect in this process. However, it is also observed that at the beginning of the cracking reaction, as shown in Figure 8, the acid value increases, proving that triacylsglicerols are broken into carboxylic acids, so the acid value increases. Afterwards, the deoxygenation of carboxylic acids occurs as reaction time succeeds, showing a drastic decrease in the acid values of OLP. In addition, as observed in Table 2, the use of RSC causes an increase in the saponification value, showing that OLP still contains saponified matter. That is, the conversion of palm oil by the catalytic cracking reaction was not complete. The low acid values are due to the acid-base neutralization and the deoxygenation/decarboxylation of carboxylic acids.

A more detailed examination of Figure 8 reveals a trend related to carboxylic acids (free fatty acids) in crude palm oil during the initial 10 min of the reaction. Significant deoxygenation is observed during this period due to the immediate and direct influence of using 20% (w/w) fresh sodium carbonate as a catalyst. Sodium carbonate plays a vital role in the deoxygenation of carboxylic acids, resulting in a substantial reduction in the content of oxygenated compounds, especially carboxylic acids, within the acid phase. Consequently, the acid value of the OLP is significantly reduced [13]. Between 10 and 40 min of reaction time, the acid value shows only minor variations and remains below 1.5 mg KOH/g. Beyond 40 min of reaction, the acid value stabilizes, indicating that the deoxygenation process reaches a point of equilibrium, and a further reduction in acid value is limited. This observation reinforces the importance of controlling the reaction time to achieve the desired reduction in acid value without compromising the overall quality of the OLP.

The experiments that used 10% (w/w) of fresh sodium carbonate and 10% (w/w) of reused sodium carbonate did not produce the same effect on the acid value of their OLPs when compared with the OLPs of the experiment that used 20% (w/w) of fresh sodium carbonate. According to Figure 8, in the first 20 min, there was an increase in the acid values, probably due to the breakdown of the crude palm oil molecules and, subsequently formation of compounds with a predominance of carboxyl [66,67,68,69]. Only after 30 min of reaction there is a decrease in the acid values mentioned above, in such a way that between 30 and 60 min of reaction, the acid values vary slightly and do not exceed 2.75 mg KOH/g for OLP obtained with 10% (w/w) FSC and 1.92 mg KOH/g for OLP obtained with 10% (w/w) RSC. This behavior in the period of 30 to 60 min is the result of the deoxygenation of carbonyls and carboxyl’s from the primary cracking of triglycerides, giving rise to gaseous products, paraffin, and olefins of long and short chains, CO, CO₂, water, and alcohols [66,67,68,69]. In this context, it is essential to highlight the performance of reused sodium carbonate, because even without any regeneration stage, it produced an OLP with a lower acid value than that obtained for the OLP from the cracking of palm oil with fresh sodium carbonate and in the same percentage. This results in yet another parameter that indicates the excellent catalytic activity of sodium carbonate after a reuse cycle compared to fresh sodium carbonate.

In Figure 9, we observe that during the initial 10 min of the reaction, there is no significant change in the saponification value for the OLP obtained with 20% (w/w) of FSC (fresh sodium carbonate) as the catalyst. However, between 10 and 20 min of the reaction, there is a notable reduction in the saponification value, stabilising after 20 min of cracking. The saponification value indicates the potassium hydroxide required to neutralize the free fatty acids and saponify the esters in one gram of fat. This measurement informs us about the presence of carboxylic acids in both free fatty acid and ester forms. The observed reduction in saponification value between 10 and 20 min suggests a reaction of cracking involving both primary and secondary cracking of palm oil. Primary cracking results in the breakdown of triglyceride molecules on the catalyst surface, producing bio-hydrocarbons and oxygenated compounds such as fatty acids, ketones, aldehydes, and esters [66,67,68,69].

During the secondary cracking, these compounds are converted into gaseous products, paraffin, olefins of different chain lengths, CO, CO₂, water, and alcohol. This secondary cracking process leads to the deoxygenation of oxygenated compounds, particularly carboxylic acids. The reduction in the saponification value indicates that the conversion of these oxygenated compounds into other products takes place within the first 20 min of the reaction. This sequential process of primary and secondary cracking highlights the intricate transformations occurring during the thermal catalytic cracking of palm oil, which ultimately results in the production of bio-hydrocarbons with improved properties and reduced oxygenated compounds, making them suitable as potential biofuels.

The experiments that used 10% (w/w) of fresh sodium carbonate and 10% (w/w) of reused sodium carbonate did not produce the same behavior in the saponification value of their OLPs when compared to the experiment that used 20% (w/w) of fresh sodium carbonate. According to Figure 9, in the first 20 min, there is an increase in the saponification values for the OLP obtained with 10% (w/w) of FSC. With 30 min of reaction time, the saponification value shows a decrease, which is due to the initial formation of linked and lower molecular weight carboxylic acids due to the intense primary cracking reaction, which corresponds to the molecular breakdown and not the secondary reaction, characterized by the deoxygenation reactions of these new molecules [66,67,68,69]. Deoxygenation reactions are intensified after 30 min, as explained by the analysis of the behavior of the acid values (see Figure 8). On the other hand, the OLP obtained with 10% (w/w) of RSC has its saponification value reduced in the first 10 min of reaction, significantly reducing between 10 and 30 min. After 30 min of reaction, the saponification values increase until the cracking process ends with 60 min of reaction.

Table 3. Refractive index and corrosiveness to the copper of the OLP collected at different times during the thermal catalytic cracking experiment of crude palm oil.

Time (Min)	Catalyst [% wt.]
	10 (FSC)		20 (FSC)		10 (RSC)
	R.I.	Corrosiveness to Copper, 3 h at 50 °C	R.I.	Corrosiveness to Copper, 3 h at 50 °C	R.I.	Corrosiveness to Copper, 3 h at 50 °C
0	1.45	1A	1.44	1A	1.45	1A
10			1.46	n.r.	1.44	n.r.
20			1.42	1A	1.45	1A
30			1.44	1A	1.46	1A
40			1.44	1A	1.45	1A
50			1.45	1A	1.45	1A
60			1.45	1A	1.45	1A

n.r., not rated. R.I., Refractive index.

reveals significant changes in the OLP’s refractive index (RI) during the 60-min thermal catalytic cracking, while no changes were observed in its corrosiveness to copper. The lowest RI value was reached at 20 min of reaction, indicating that within the interval of 10 to 20 min, a beneficial cracking reaction occurred, leading to the formation of smaller molecules compared to the size of the feedstock molecules. According to Canapi [80] and Swern [81], there is a general relation between the refractive index and the composition of oil-based products, where the refractive index increases as the length of the carbon chain increases. Therefore, the decrease in RI during the first 20 min suggests the formation of shorter carbon-chain molecules due to the cracking process. However, after 20 min, the refractive index gradually increased, although it did not exceed the RI value of the original palm oil (RI = 1.46, as explained by Mota et al. [23]). This indicates that, from the 20-min mark, oligomerization reactions started to take place, leading to the formation of larger molecules. Ong and Bhatia [82] have suggested that light alkenes can undergo oligomerization to produce a mixture of alkenes and heavier alkanes, which are then fractionated into gasoline, kerosene, and diesel. The fractional distillation results of the OLP obtained in the present study (Section 3.6.2) indeed confirm that it is composed of biofuels falling within the range of gasoline, kerosene, and diesel. This suggests that the thermal catalytic cracking process effectively produces a range of biofuels suitable for various applications in the transportation sector.

3.5.2. FTIR Spectroscopy

To study the quality of OLPs as a function of the reaction time and the cracking reaction, the infrared spectrum of OLPs from the experiment that employed 10% (w/w) of fresh sodium carbonate was analyzed, as shown in Figure 11. Figure 11 shows that with 20 min of cracking process, bands such as 1748 cm⁻¹ disappear, indicating the breakdown of triglyceride molecules and, consequently, the formation of oxygenated compounds such as carboxylic acids, aldehydes, and ketones [65,66,67,68,69]. Carboxylic acids are confirmed by the presence of a band in the range 3500 to 2500 cm⁻¹, referring to the O-H bond, which according to Silverstein [74] and Pavia [75], corresponds to the function of carboxylic acids, whose appearance occurs through the presence of a broad and extensive band in the range of 3400 to 2400 cm⁻¹. Associated with this band are the intense peaks of 1708 cm⁻¹ and 1707 cm⁻¹ corresponding to the group C=O, which appears in the range of 1730–1700 cm⁻¹, more specifically close to or over 1710 cm⁻¹, referring to carboxylic acids aliphatic [74,75]. The presence of carboxylic acids is also confirmed by the band 1284 cm⁻¹ corresponding to the stretching of the C-O bond. Ketones and aldehydes are confirmed due to characteristic bands such as 1716 cm⁻¹, 1718 cm⁻¹, 1724 cm⁻¹, and 2729 cm⁻¹, corresponding to normal aliphatic aldehydes and ketones [74,75]. In addition, as the cracking time increases, the broadband in the range from 3500 to 2500 cm⁻¹, becomes increasingly narrower, focusing near or over the 3000 cm⁻¹ positions. Simultaneously, the bands move from right to left, that is, from the lowest to the highest value, such as the disappearance of the band 1284 cm⁻¹ and consequently the appearance of bands close to or equal to 1375 cm⁻¹, with a relative increase in intensity in the respective band, indicating the decomposition of oxygenated compounds, and with this the formation of alkanes or saturated hydrocarbons [66,67,68,69]. It is important to highlight the presence of bands in the range from 1000 cm⁻¹ to 700 cm⁻¹, which confirm the presence of olefins and aromatic compounds, indicating that oligomerization reactions of the products formed during the cracking process may have occurred as reaction time increases [82].

3.6. Influence of the Fractional Distillation Process on the Yield and Quality of Biofuels

3.6.1. Mass Balances of the Distillation Process

The fractional distillation process yields a little more than 62% (w/w) of green gasoline/Range 1, green gasoline/Range 2, green aviation kerosene and green diesel from the OLP produced by thermal catalytic cracking of crude palm oil with 20% (w/w) sodium carbonate. In such a way, sodium carbonate yields a higher amount of green aviation kerosene, followed by green diesel and green gasoline. Similar results were reported by Kurniawan et al. [83], which produced a range of bio-hydrocarbons from the catalytic pyrolysis of coconut oil soap, most of which proved to be similar to jet fuel.

According to

Table 4. Process parameters and mass balance for the fractional distillation on a laboratory scale.

Process Parameters	Unit	Values
The initial mass of OLP	g	653.37
Initial operating temperature	°C	31
Initial distillation temperature	°C	102
Final distillation temperature	°C	400
Mass of non-condensable gases	g	37.17
Mass of green gasoline/Range 1	g	31.69
Mass of green gasoline/Range 2	g	62.00
Mass of green aviation kerosene	g	157.85
Mass of green diesel	g	155.84
Mass of bottoms product	g	208.82
Yield of non-condensable gases	% wt.	5.69
Yield of green gasoline/Range 1	% wt.	4.85
Yield of green gasoline/Range 2	% wt.	9.49
Yield of green aviation kerosene	% wt.	24.16
Yield of green diesel	% wt.	23.85
Yield of bottoms product	% wt.	31.96

, the OLP obtained through the thermal catalytic cracking of crude palm oil with 20% (w/w) sodium carbonate yielded four distinct fractions during fractional distillation at different temperature ranges. These fractions were categorized as previously defined: B.P < 90 °C (green gasoline/Range 1), 90 ≤ B.P ≤ 160 °C (green gasoline/Range 2), 160 ≤ B.P ≤ 245 °C (green aviation kerosene), and 245 ≤ B.P ≤ 340 °C (green diesel). The results in Table 4 indicate that as the distillation temperature range increased, the yield of the corresponding fractions also increased. This suggests that the thermal catalytic cracking process with sodium carbonate favored the production of heavier fractions, such as green aviation kerosene, green diesel, and the bottoms product. It appears that sodium carbonate played a selective role in promoting bio-hydrocarbons formation within the distillation temperature range associated with green aviation kerosene and green diesel.

Similar findings were reported by Weber et al. [42] when they conducted the thermal degradation of animal fat using sodium carbonate as the catalyst at pilot-scale temperatures between 410 and 450 °C, along with 5% (w/w) of water. They subsequently performed fractional distillation of the resulting OLP to obtain fractions within the boiling temperature range of gasoline and diesel oil. Their study revealed that the fractional distillation of the OLP yielded 66% (w/w) of the diesel fraction and 21% (w/w) of the gasoline fraction.

The total yield obtained from the sum of the four fractions (green gasoline/Range 1, green gasoline/Range 2, green aviation kerosene, and green diesel) is 62.35% (w/w), which is higher than what has been reported in some literature studies. For instance, Chew & Bhatia [84] investigated the catalytic cracking of crude palm oil and used palm oil using different additives (HZSM-5 with varying Si/Al ratios, beta zeolite, SBA-15, and AlS-BA-15) physically mixed with Rare Earth-Y (REY) as a catalyst. Their best results using HZSM-5 (Si/Al ratio = 40) with REY as a catalyst showed that OLP consisted of 59.3% and 55.3% (w/w) of fractions in boiling temperature ranges of gasoline, kerosene, and diesel oil, respectively, for crude palm oil and used palm oil. However, these values are lower than the total yield obtained in the present study.

Furthermore, the bottom product obtained from the fractional distillation of the OLP showed the highest yield in the current study. This bottom product contains a diverse range of valuable chemicals, making it potentially useful in various applications such as resins, agrochemicals, fertilizers, emission control agents [47], fuels [58,85], carbon anodes, steel carburization, and graphite synthesis [86]. The results presented in Table 4 demonstrate that OLP obtained through thermal catalytic cracking of crude palm oil with 20% (w/w) fresh sodium carbonate at 450 °C, 1 atm, and at a pilot scale yielded a higher amount of distilled fractions compared to OLP produced using commercial catalysts such as zeolites. Moreover, the bottom product formed during the fractional distillation process of the initial OLP can be utilized in diverse applications, highlighting the potential value of this method in the production of biofuels and valuable chemicals.

3.6.2. Characterization of the Distilled Fractions

Physical-Chemical Properties

The fractional distillation at 1 atm of the OLP improves the values of the physical-chemical properties of the biofuels, allowing the majority of the values of the physical-chemical properties of the distilled fractions to be within limits established by the Agência Nacional do Petróleo, Gás Natural e Biocombustíveis (ANP). Therefore, biofuels as distilled fractions (green gasoline/Range 1, green gasoline/Range 2, green aviation kerosene, and green diesel) may be used as partial or total substitutes for particular petroleum products.

The green gasoline (Range 1 and 2), green aviation kerosene, and green diesel fractions were characterized by physical-chemical analyzes as described in Section 2.5.2, and the results are in

Table 5. Physical-chemical properties of the green gasoline (Range 1) and green gasoline (Range 2) fractions.

Physical-Chemical Properties	Unit	OLP [23]	Green Gasoline		ANP № 40 [87]	ASTM D4814 (Gasoline/Type A) [88]	Gasoline Fraction [19]	Gasoline Fraction [50]
			Range 1	Range 2
Specific gravity at 20 °C	kg/m3	790.00	690.00	750.00	Annotate	-	866.00	843.80
Kinematic viscosity at 40 °C	mm2/s	2.02	0.72	0.76	-	-	2.34	-
Flashpoint, min	°C	85.10	2.10	3.00	-	-	34.00	-
Corrosiveness to copper, 3 h 50 °C, max.	-	1	1	1	1	1	-	-
Carbon residue, max	wt.%	0.64	n.r.	-	-	-	-	-
Acid value	mg KOH/g	1.02	1.11	1.43	-	-	2.30	7.60
Saponification value	mg KOH/g	14.35	12.96	14.29	-	-	-	-
Refractive index	-	1.44	1.40	1.42	-	-	-	-
Ester value	mg KOH/g	13.33	11.85	12.86	-	-	-	-
Content of FFA	wt.%	0.51	0.55	0.72	-	-	-	-
Distillation
10% vol., recovered, max.	°C	n.r.	72.90	116.10	65.00	70.00	-	-
50% vol., recovered, max.		n.r.	102.10	144.60	120.00	121.00	-	-
90% vol., recovered		n.r.	174.50	196.80	190.00	190.00	-	-
Final Boiling Point (F.B.P), max.		n.r.	214.5	222.30	215.00	225.00	-	-

viscosity at 20 °C; FFA, free fatty acids; n.r., not rated; max, maximum; min, minimum; ANP: Agência Nacional do Petróleo, Gás Natural e Biocombustíveis, Resolution № 40 (Specification of regular gasoline/Type A).

,

Table 6. Physical-chemical properties of the green aviation kerosene fraction.

Physical-Chemical Properties	Unit	OLP [23]	Green Aviation Kerosene	ANP № 37 (Aviation Kerosene) [89]	ASTM D1655 (Jet A-1) [90]
Specific gravity at 20 °C	kg/m3	790.00	790.00	771.30–836.60	775–840 a
Kinematic viscosity at 40 °C	mm2/s	2.02	1.48	-	8.0 b
Flashpoint, min	°C	85.10	10.00	38.00	38.00
Corrosiveness to copper, 3 h 50 °C, max.	-	1	1	-	1 c
Carbon residue, max.	wt.%	0.64	0.02	-	-
Acid value	mg KOH/g	1.02	1.68	0.015 d	0.10 d
Saponification value	mg KOH/g	14.35	15.05	-	-
Refractive index	-	1.44	1.44	-	-
Ester value	mg KOH/g	13.33	13.37	-	-
Content of FFA	wt.%	0.51	0.84	-	-
Distillation
Initial Boiling Point (I.B.P),	°C	n.r.	-	Annotate	-
10% vol., recovered, max.		n.r.	186.60	205.00	205.00
50% vol., recovered		n.r.	221.80	Annotate	Annotate
90% vol., recovered		n.r.	259.10	Annotate	Annotate
Final Boiling Point (F.B.P), max.		n.r.	278.60	300.00	300.00
(90% vol., recovered) T90—(10% vol., recovered) T10, min.		n.r.	72.5	-	-

FFA, free fatty acids; n.r., not rated; max, maximum; min, minimum; ANP: Agência Nacional do Petróleo, Gás Natural e Biocombustíveis, Resolution № 37 (Specification of Aviation kerosene). ^a Density at 15 °C, kg/m³. ^b Viscosity −20 °C, mm²/s (max.). ^c Copper strip, 2 h at 100 °C (max.). ^d ASTM Test Method: ASTM D3242.

and

Table 7. Physical-chemical properties of the green diesel fraction.

Physical-Chemical Properties	Unit	OLP [23]	Green Diesel	ANP № 65 (Diesel) [73]	ASTM D975 (Diesel № 1-D S15) [79]	Diesel Fraction [19]	Diesel Fraction [50]
Specific gravity at 20 °C	kg/m3	790.00	820.00	820–850	-	898	881.90
Kinematic viscosity at 40 °C	mm2/s	2.02	3.51	2–4.50	1.3–2.4	6.27 a	-
Flashpoint, min	°C	85.10	24.00	38.00	38.00	115	-
Corrosiveness to copper, 3 h 50 °C, max	-	1	1	1	3	-	-
Carbon residue, max	wt.%	0.64	-	0.25	0.15 b		-
Acid value	mg KOH/g	1.02	5.70	0.50	-	2.50	86.90
Saponification value	mg KOH/g	14.35	15.80	-	-	-	-
Refractive index	-	1.44	1.45	-	-	-	-
Ester value	mg KOH/g	13.33	10.10	-	-	-	-
Content of FFA	wt.%	0.51	2.87	-	-	-	-
Distillation
10% vol., recovered	°C	n.r	260.20	180	-	-	-
50% vol., recovered		n.r	282.30	245–295	-	-	-
85% vol., recovered, max.		n.r	326.57	-	-	-	-
90% vol., recovered, max.		n.r	332.90	-	288.00	-	-
95% vol., recovered, max.		n.r	353.30	370	-	-	-

^aviscosity at 20 °C; FFA, free fatty acids; n.r., not rated; max, maximum; min, minimum; ANP, Agência Nacional do Petróleo, Gás Natural e Biocombustíveis, Resolution № 65 (Specification of Diesel S10). ^bASTM Test Method: ASTM D524.

. These results were compared with the values of the physical-chemical properties of the OLP and petroleum products (gasoline, kerosene, and diesel).

As shown in Table 5, most of the values concerning the physical-chemical properties of green gasoline fractions (Range 1 and 2) were considerably lower than the values of the physical-chemical properties of the OLP. These values are also lower than those found in the works of Xu et al. [19] and Wisniewski Jr. et al. [50] for gasoline fractions. Besides, the experimental data for the green gasoline fraction (Range 1) are in agreement with the specifications defined for the regular gasoline (Type A) specification of ANP № 40 [87] and gasoline (Type A) specification of ASTM D4814 [88], mainly about the recovered volume. This indicates that the green gasoline fraction (Range 1) can be used as a fuel for Otto-cycle motor vehicles, acting as a partial or total replacement for petroleum gasoline.

Table 5 shows that the recovered volume of green gasoline (Range 2) fraction obtained in this study is not in agreement with the specifications established since the reference temperatures exceed the limits specified for the respective recovered volume, according to the rules of the ANP № 40 [87] and ASTM D4814 [88]. However, this disagreement can be resolved by blending the green gasoline (Range 1) with green gasoline (Range 2) or performing fractional distillation under a distillation temperature range of less than 160 °C to obtain a single fraction of green gasoline that meets the limits fixed or the regular gasoline (Type A) specification of ANP № 40 [87] or gasoline (Type A) specification of ASTM D4814 [88].

Table 6 indicates that most of the physical-chemical properties of the green aviation kerosene fraction either match or are lower than those of the original OLP. However, the acid value obtained for the green aviation kerosene fraction was 1.68 mg KOH/g, which, although relatively low, has not reached the limit set by ANP № 37 [89] and ASTM D1655 [90]. To meet the regulatory requirements, further reduction of the acid value can be achieved through additional separation processes such as liquid-liquid extraction and adsorption [53,91] or fractional distillation [53,62]. These techniques can potentially bring the acid value within the acceptable limits of ANP and ASTM for aviation kerosene.

The flash point value was lower than that established by the ANP № 37 [89] and ASTM D1655 [90], as shown in Table 6. This fact is due to low molecular weight compounds and lack of adequate normal paraffinic and aromatic compounds, resulting in a lower flash point for the green aviation kerosene fraction. The flash point indicates the presence of volatile components in the oil and is used to evaluate the overall flammability hazard of a material. The lower the flash point, the higher the concentration of light bio-hydrocarbons in the material [92]. This fact can be corroborated in section “GC–MS Analysis”, which corresponds to the chromatographic analysis results for the distilled fractions, including the fraction of green aviation kerosene. Also, the specific gravity and recovered volume of green aviation kerosene are in agreement with the specifications established for aviation kerosene according to ANP № 37 (Specification of Aviation Kerosene) [89] and ASTM D1655 (Jet A-1) [90].

Table 7 reveals that most of the physical-chemical properties of the green diesel fraction were higher than those found in the original OLP. However, it is worth noting that the specific gravity and acid value of the green diesel fraction were lower than those reported in the studies by Xu et al. [19] and Wisniewski Jr. et al. [50]. The properties such as specific gravity at 20 °C, Kinematic Viscosity at 40 °C, corrosiveness to copper, and recovered volume of the green diesel fraction are in line with the specifications set for Diesel S10 by ANP № 65 [73]. Nevertheless, properties like flash point and acid value did not meet the limits established by the regulatory agency. The lower flash point value in the green diesel fraction can be attributed to low molecular weight compounds and a lack of an adequate percentage of normal paraffinic and aromatic compounds, as discussed in Section 3.5.1, resulting in a reduced flash point value for the green diesel fraction. Further optimization and refining processes may be required to meet the flash point and acid value specifications of ANP and ASTM for diesel fuels.

The flash point of a fuel is vital for safety, as it is necessary for legal and safety precautions in its handling and storage [93]. Removal of volatile components through an upgrade can quickly improve the flash point [54,94,95], making it possible for distillate fractions, especially green aviation kerosene and green diesel, to reach the limits set by regulatory agencies.

The acid value of the green diesel fraction has not reached the limit established by the ANP № 65 [73], and the value obtained for this property was equal to 5.70 mg KOH/g, which is higher than the acid value of the other distilled fractions. This fact results from the higher concentration of carboxylic acids in green diesel fraction than other distilled fractions, as indicated in

Table 8. Retention time, chemical compounds, and peak areas for the compounds identified by GC-MS analysis for the green diesel fraction.

Retention Time (min)	Molecular Formula	Chemical Compounds	Area (%)
6.483	C6H12O2	Diacetone alcohol	1.12
6.599	C13H28	Tridecane	3.40
6.964	C14H28	1-Tetradecene	2.39
8.067	C14H30	Tetradecane	5.29
8.461	C15H30	1-Pentadecene	7.91
9.562	C15H32	Pentadecane	11.23
9.926	C16H32	1-Hexadecene	18.81
10.014	C16H34O	1-Hexadecanol	2.25
10.606	C16H32	Decylcyclohexane	1.90
10.776	C16H34	Hexadecane	4.21
11.138	C17H34	1-Heptadecene	6.13
11.265	C17H36O	1-Heptadecanol	2.81
11.408	C17H36	Heptadecane	2.11
11.998	C18H34	1-Octadecyne	5.78
12.371	C18H36	1-Octadecene	7.38
12.478	C20H38	1,19-Eicosadiene	4.71
16.598	C17H34O	2-Heptadecanone	7.88
17.229	C18H36O	4-Octadecanone	1.83
18.48	C19H38O	2-Nonadecanone	1.88
20.74	C15H30O2	Pentadecanoic acid	0.98

and

Table 9. Chemical composition of distilled fractions in terms of hydrocarbons and oxygenates compounds.

Product Groups	Area (% wt.)
	OLP [13]	Green Gasoline		Green Aviation Kerosene	Green Diesel
	20% (w/w) Na2CO3	Range 1	Range 2
Hydrocarbons	88.10	50.14	93.64	93.97	81.25
Normal paraffin	24.28	13.80	20.52	35.91	24.13
Olefins	51.74	32.93	45.28	53.90	55.22
Naphthenic	12.08	3.41	25.35	4.16	1.90
Aromatics	0.00	0.00	2.49	0.00	0.00
Oxygenates	11.90	49.86	6.36	6.03	18.75
Carboxylic acids	3.10	0.00	0.00	0.00	0.98
Alcohols	3.31	7.21	0.00	3.66	5.06
Ketones	5.49	15.13	3.95	1.26	11.59
Esters	0.00	14.53	0.00	0.00	0.00
Others	0.00	12.99	2.41	1.11	1.12
Total	100.00	100.00	100.00	100.00	100.00

. Finally, according to Table 7, the recovered volume of green diesel fraction obtained agrees with the limits established for the Diesel S10 specification of ANP № 65 [73], except for the temperature related to 10% of recovered volume.

FTIR Spectroscopy

Figure 12 presents the FTIR spectra collected for OLP and its distilled fractions. The IR spectrum serves as a preliminary analysis to define what chemical functions are observed. It can be confirmed in the GC-MS analysis by showing specific vibrations associated with the presence of determined chemical bonds and functional groups.

The FTIR spectra of distilled fractions show the axial deformation of C-H bonds, characteristic of organic compounds as hydrocarbons, in the region of 3000–2840 cm⁻¹. Angular deformation vibration of methyl groups is also observed in the 1375 cm⁻¹ range, indicating the presence of linear hydrocarbons. The presence of carboxylic acids, ketones and aldehydes can be suggested by the vibration band between 1716–1722 cm⁻¹, corresponding to carbonyl C-O bonds.

Distillation Curve

The results concerning the distillation curves of the biofuels in the form of distilled fractions such as green gasoline, green aviation kerosene, and green diesel show that they can be used as one of the fractions of the “pool” of biofuel to replace form partial or total an oil derivative. Also, the results obtained for the four fractions distilled regarding the similarity of the experimental distillation curves with standard distillation curves come from the OLP quality, fractional distillation efficiency, and temperature range selected to obtain each fraction.

Figure 13, Figure 14 and Figure 15 show the distillation curves of distilled fractions. The figures exhibit the chemical and physical behavior of bio-hydrocarbon fractions present in the biofuel obtained in this study within the following temperature ranges: DT < 90 °C (green gasoline fraction/Range 1); 90 ≤ DT ≤ 160 °C (green gasoline fraction/Range 2), 160 ≤ DT ≤ 245 °C (green aviation kerosene fraction), 245 ≤ DT ≤ 340 °C (green diesel fraction).

In Figure 13, the experimental distillation curves of the green gasoline fractions (Range 1 and 2) display similarities to the standard distillation curve for regular gasoline (Type A) as specified by ANP № 40 [87], especially the distillation curve of green gasoline fraction (Range 1). Although the distillation temperature for the 10% recovered volume exceeds the standard value, the temperatures for other recovered volumes (50%, 80%, 90%, and 100%) are consistent with the standard distillation curve for regular gasoline derived from petroleum, as per ANP № 40 [87]. Similar results were obtained by Wiggers et al. [96], who fractionated soybean bio-oil into light distilled fractions with distillation curves resembling those of petroleum-based gasoline.

Figure 14 indicates that the green aviation kerosene fraction meets the limits set by the distillation curve for aviation kerosene according to ANP № 37 [89]. This suggests that the green aviation kerosene fraction can be utilized as fuel when blended with commercial petroleum products. Ertas and Alma [97] also obtained simulated distillation curves for the kerosene fraction from bio-oil derived from laurel (Laurus nobilis L.). They found it suitable for use as fuel when mixed with petroleum products.

Figure 15 shows that the experimental distillation curve of the green diesel fraction differs from the standard distillation curve for Diesel S10 specification (ANP № 65) [73] up to the 40% recovered volume. In this range, the distillation temperatures are higher than the limits established by ANP, indicating an excessive concentration of heavy bio-hydrocarbons in the green diesel fraction compared to petroleum diesel. However, the temperatures related to the 50% and 95% recovered volumes are consistent with the standard distillation curve, suggesting that the green diesel fraction can be utilized as fuel when blended with petroleum products. Similar findings were reported by Ertas and Alma [97], who obtained simulated distillation curves for the diesel fraction from bio-oil derived from laurel (Laurus nobilis L.) and also concluded that it can be used as fuel when mixed with petroleum products.

GC–MS Analysis

The GC-MS analysis of the distilled fractions is presented through chromatograms, as depicted in Figure 16, Figure 17, Figure 18 and Figure 19. Table 8 and

Table 10. Retention time, chemical compounds, and peak areas for the compounds identified by GC-MS analysis for the green gasoline fraction (Range 1).

Retention Time (min)	Molecular Formula	Chemical Compounds	Area (%)
3.044	C6H12O	2-Hexanone	4.07
3.183	C8H12	2,5,5-Trimethyl-1,3-cyclopentadiene	0.54
3.397	C9H16	1,8-Nonadiene	0.81
3.621	C10H20	Butylcyclohexane	2.87
3.948	C11H24	Undecane	10.79
4.096	C7H14O	2-Heptanone	7.43
4.232	C13H26	1-Tridecene	13.98
4.342	C14H26	1-Tetradecyne	3.01
4.392	C11H24O	1-Undecanol	4.98
4.525	C14H28	(7E)-7-Tetradecene	4.86
5.175	C14H28	(5E)-5-Tetradecene	6.31
5.683	C14H30	Tetradecane	0.45
6.485	C6H12O2	Diacetone alcohol	12.99
6.903	C13H28O	1-Tridecanol	2.23
7.015	C9H18O	2-Nonanone	1.88
7.909	C15H32	Pentadecane	1.37
8.311	C16H32	(3Z)-3-Hexadecene	1.90
8.504	C10H20O	2-Decanone	0.80
9.249	C16H34	Hexadecane	1.19
9.657	C17H34	1-Heptadecene	1.69
12.142	C19H38	1-Nonadecene	0.38
16.5	C15H30O	2-Pentadecanone	0.95
16.819	C17H34O2	Hexadecanoic acid, ethyl ester	7.27
18.554	C16H22O4	Diisobutyl phthalate	0.47
18.917	C18H34O2	Ethyl Oleate	5.97
19.21	C20H36O2	9,12-Octadecadienoic acid, ethyl ester	0.82

,

Table 11. Retention time, chemical compounds, and peak areas for the compounds identified by GC-MS analysis for the Green gasoline fraction (Range 2).

Retention Time (min)	Molecular Formula	Chemical Compounds	Area (%)
3.511	C8H10	Ethylbenzene	2.49
4.042	C10H18	9-Methylbicyclo [3.3.1] nonane	8.98
4.268	C11H22	1-Undecene	16.80
4.422	C11H22	1-Pentyl-2-propylcyclopropane	15.17
5.214	C11H24	Undecane	9.93
5.531	C12H24	1-Dodecene	15.09
5.710	C12H24	(2Z)-2-Dodecene	1.86
5.868	C12H24	Nonylcyclopropane	1.20
6.484	C6H12O2	Diacetone alcohol	2.41
6.561	C13H28	Tridecane	7.47
6.931	C14H28	1-Tetradecene	4.91
7.016	C9H18O	2-Nonanone	2.61
7.923	C14H30	Tetradecane	2.01
8.326	C15H30	1-Pentadecene	3.93
8.496	C10H20O	2-Decanone	1.34
9.253	C16H34	Hexadecane	1.10
9.661	C17H34	1-Heptadecene	2.71

and

Table 12. Retention time, chemical compounds, and peak areas for the compounds identified by GC-MS analysis for the green aviation kerosene fraction.

Retention Time (min)	Molecular Formula	Chemical Compounds	Area (%)
4.069	C10H22	Decane	3.34
4.293	C11H22	1-Undecene	6.78
4.448	C11H22	1-Pentyl-2-propylcyclopropane	4.15
5.292	C12H26	Dodecane	6.26
5.610	C13H26	1-Tridecene	9.42
5.924	C12H26O	1-Dodecanol	1.10
6.492	C6H12O2	Diacetone alcohol	1.11
6.713	C13H28	Tridecane	11.30
7.057	C14H28	1-Tetradecene	10.53
7.206	C14H28	(7E)-7-Tetradecene	1.26
7.958	C15H32	Pentadecane	7.23
8.503	C16H32	1-Hexadecene	14.58
9.300	C17H36	Heptadecane	6.08
9.855	C18H36	(3E)-3-Octadecene	9.10
10.622	C18H38	Octadecane	1.26
11.012	C19H38	1-Nonadecene	2.22
11.186	C14H28O	(Z)-9-Tetradecen-1-ol	1.03
11.780	C19H40	Nonadecane	0.44
12.180	C16H34O	1-Hexadecanol	0.97
12.319	C18H36O	Oleyl Alcohol	0.56
12.495	C13H26O	2-Tridecanone	0.57
16.501	C15H30O	2-Pentadecanone	0.69

provide information about the retention time, chemical compounds, and peak areas of the identified compounds for each distilled fraction. The components present in the distilled fractions are classified into two significant groups: bio-hydrocarbons (including normal paraffinic, olefin, naphthenic, and aromatic compounds) and oxygenates (comprising carboxylic acids, alcohols, aldehydes, ketones, esters, and others). Upon analyzing Figure 16, Figure 17, Figure 18 and Figure 19, it becomes evident that there are numerous peaks, indicating a high number of compounds in each distilled fraction. Figure 16 specifically illustrates that the compounds in the green gasoline (Range 1) fraction are predominantly located within the interval of 2.5 to 7.5 min and 15 to 20 min, suggesting the presence of both long and short bio-hydrocarbon chains, as indicated in Table 10. However, it is noteworthy that short bio-hydrocarbon chains are more prevalent in this fraction.

In Figure 17, the compounds of the green gasoline (Range 2) fraction are distributed between 2.5 and 10 min. However, the most intense peaks are observed between 2.5 and 5.0 min, which indicates the presence of peaks similar to those found in the green gasoline (Range 1) fraction. Table 10 and Table 11 further confirm the presence of compounds such as di-acetone alcohol, 2-nonanone, 2-decanone, and hexadecane in both fractions (green gasoline/Range 1 and green gasoline/Range 2).

In Figure 18, the green aviation kerosene fraction compounds are primarily found between 3 and 12.5 min, suggesting the presence of intermediate bio-hydrocarbon chains. The most intense peaks are observed between 7.5 and 10 min, where notable peaks correspond to the retention times of 8.503 min (1-Hexadecene) and 9.855 min [(3E)-3-Octadecene], as listed in Table 12.

Figure 19 indicates that the compounds of the green diesel fraction are distributed between 5 and 22.5 min, pointing to the presence of a wide range of bio-hydrocarbon chains. However, unlike the green gasoline and green aviation kerosene fractions, there is a concentration peak between 10 and 17.5 min, suggesting the presence of long-chain compounds, as reflected in Table 8.

Figure 20 displays the presence of bio-hydrocarbon chains ranging from C8 to C23 in all four distilled fractions. However, each fraction has its specific range of bio-hydrocarbon chains. The green gasoline fraction (Range 1) consists of bio-hydrocarbons with chains in the C8–C19 range, with higher concentrations of bio-hydrocarbons ranging from C9 to C13. Similarly, the green gasoline fraction (Range 2) comprises bio-hydrocarbons with chains in the C8–C17 range, with higher concentrations of bio-hydrocarbons with chains in the C10–C12 range. These findings are consistent with the literature, where gasoline is known to consist of bio-hydrocarbons with chains ranging from C4 to C12 [92,98], making the green gasoline fractions (Ranges 1 and 2) align with these established limits for bio-hydrocarbon chain lengths.

The green aviation kerosene fraction consists of bio-hydrocarbons with chain lengths ranging from C10 to C21, with the predominant bio-hydrocarbons falling within the C11-C16 range. These results are consistent with those reported by Speight [92], where kerosene-type jet fuels (JP-4) are characterized by hydrocarbon chains in the C4–C16 range. Unlike the distilled fractions corresponding to green gasoline (Ranges 1 and 2), green aviation kerosene contains a higher proportion of long-chain hydrocarbons. On the other hand, the green diesel fraction consists of hydrocarbons with chain lengths ranging from C13 to C23, with most hydrocarbons falling within the C14–C17 range. Speight [92] also reports that the carbon number limit for diesel fuel is typically C8–C18. Thus, the hydrocarbon chain lengths of green aviation kerosene and green diesel fraction align with the literature’s descriptions for these respective petroleum derivatives.

In a previous study by Mancio et al. [13], bio-oil was produced from the thermal catalytic cracking of palm oil with 20% (w/w) Na₂CO₃ as the catalyst, using the same equipment and operating conditions as in the current study. The GC-MS analysis results for that OLP are presented in Table 9 for comparison with the results obtained in this study for the distilled fractions. According to Table 9, the chemical composition of bio-hydrocarbons and oxygenated compounds in the four distilled fractions significantly differs from the OLP. In the OLP, bio-hydrocarbons were more abundant than oxygenated compounds, with olefins being the most predominant bio-hydrocarbon class, followed by normal paraffins and naphthenic compounds. Among the oxygenated compounds, ketones had the highest levels, followed by alcohols and carboxylic acids.

Similarly, Table 9 reveals that all the biofuels produced in the current study in distilled fractions have higher amounts of bio-hydrocarbons than oxygenated compounds. The fractions with the highest bio-hydrocarbon contents are green gasoline fraction (Range 2) and green aviation kerosene fraction, resulting in lower oxygenated compound contents for these distilled fractions. Among the identified and quantified bio-hydrocarbons are normal paraffins, olefins, naphthenic compounds, and aromatics, which are also the main components present in distilled fractions obtained from crude oil, as reported by Speight [98] and Fahim et al. [93]. Regarding the content of aromatic compounds, the green gasoline fractions (Ranges 1 and 2) comply with the ANP № 40 [87], which specifies that regular gasoline must not contain more than 25% (w/w) of aromatic compounds in its composition.

The distribution of oxygenated compounds among the four distilled fractions indicates that the green gasoline (Range 1) and green diesel fractions contain the highest levels of these components (as shown in Table 9). It is expected that some oxygenates may not be completely converted into bio-hydrocarbons during the triglyceride cracking reactions, as one of the dominant steps in the process involves the elimination of heavily oxygenated bio-hydrocarbons, such as carboxylic acids, aldehydes, ketones, and esters [66,68]. Among the oxygenated compounds, alcohols, ketones, and esters represent the three major chemical families in both distilled fractions. According to Oasmaa [94], bio-oil acidity is influenced by compounds, including oxygenates. Among oxygenates, carboxylic acids are the class that contributes most to acidity. Therefore, the green diesel fraction with 2.51% (w/w) of carboxylic acids is the distilled fraction with the highest acid value, as shown in Table 7. The results of the GC-MS analysis demonstrate that most distilled fractions can be used as an energy source by incorporating them as bio-additives in fuels, thanks to their high bio-hydrocarbon content and low oxygen levels. These bio-additives can be utilized by emulsifying the fraction with its respective oil derivative. Additionally, the distilled fractions can serve as feedstock for further upgrading processes in a crude oil refinery, particularly through hydrotreating processes [47]. These processes can further refine the fractions and remove undesirable components, making them suitable for blending with traditional petroleum-derived fuels, thus enhancing the overall quality and properties of the resulting fuel product.

Indeed, the composition of the green gasoline fraction (Range 1) indicates room for improvement, particularly concerning the presence of naphthenic and aromatic compounds at relatively low concentrations. According to Speight [98], the presence of these compounds can influence the resistance to detonation, which is essential for the antiknock power of fuels used in spark-ignition engines, such as gasoline. However, the presence of normal paraffinic compounds in the green gasoline fraction implies greater stability and resistance to oxidation, which is essential for the durability of the fuel.

Oxygenates in the green gasoline fractions (Range 1 and 2) can also impact its volatility and energy efficiency. As explained by Speight [98], oxygenates can lower the volatility of gasoline, which can improve the fuel’s ignition timing and burning characteristics. Additionally, the presence of oxygenated compounds can increase the latent heat of vaporization, meaning that more energy is required to vaporize the liquid fuel.

On the other hand, the absence of aromatic compounds in the green aviation kerosene and green diesel fractions is considered favorable, as indicated by Speight [98]. Aromatic compounds can contribute to higher resistance to detonation in diesel engines, which could be a concern in aviation kerosene. Additionally, the presence of normal (linear) paraffinic compounds in diesel fuel promotes greater stability and resistance to oxidation. However, olefins in diesel fuel may lead to a low freezing point and a high cetane number, which can be addressed by hydrogenation processes to improve the product quality without significantly changing the boiling range. Overall, the results highlight the composition and properties of the different distilled fractions, which provide valuable insights into their potential applications as bio-additives and feedstock for further upgrading processes in the production of biofuels with improved properties and performance.

Table 13. Comparison of chemical composition (hydrocarbons and oxygenates compounds) of distilled fractions reported in the literature with that of the present study.

Distillation	Feed	Temperature	Pressure	Distilled fraction	Hydrocarbons (% Area)	Oxygenates (% Area)	References
Fractional distillation	OLP of Palm oil	<90 °C	1 atm	Green gasoline (Range 1)	56.02	43.98	This study
Fractional distillation	OLP of Palm oil	90–160 °C	1 atm	Green gasoline (Range 2)	86.37	13.63	This study
Fractional distillation	OLP of Palm oil	160–245 °C	1 atm	Green aviation kerosene	91.38	8.62	This study
Fractional distillation	OLP of Palm oil	245–340 °C	1 atm	Green diesel	70.78	29.22	This study
Reactive distillation	Bio-oil	<200 °C	1 atm	Light bio-oil	60.06	1.88	[50]
Simple distillation	Bio-oil of carinata oil	<200 °C	1 atm	Hydrocarbon biofuel	91.97	8.03	[17]
Fractional distillation	Corn stover bio-oil	≤100 °C	1 atm	Light fraction	23.1	15.9	[47]
Fractional distillation	Corn stover bio-oil	100–180 °C	1 atm	Middle fraction	24.9	14.9	[47]
Fractional distillation	Corn stover bio-oil	180–250 °C	1 atm	Heavy fraction	7.1	6.8	[47]
Fractional distillation	OLP of residual FOG a from grease traps	175–235 °C	1 atm	Kerosene fraction	94.62	5.38	[51]
Fractional distillation	OLP of residual FOG a from grease traps	235–305 °C	1 atm	Light diesel fraction	73.86	26.14	[14]
Fractional distillation	Bio-oil of deodar sawdust	<140 °C	1 atm	IBP b < 140 °C fraction	46.04	2.59	[99]
Fractional distillation	Bio-oil of pine sawdust	<140 °C	1 atm	IBP b< 140 °C fraction	42.87	1.06	[99]

^aFat, oils, and grease: FOG; ^bInitial Boiling Point: IBP.

provides a comprehensive comparison of the bio-hydrocarbon and oxygenate contents obtained from the current study for the distilled fractions with results from different research studies. It highlights the type of distillation method used, the feedstock employed, and the operating conditions of pressure and temperature. The results of the present study consistently show that the bio-hydrocarbon contents in the green gasoline fractions (Range 1 and 2) and green aviation kerosene fraction are notably higher than those reported in other studies, including the light, middle, and heavy fractions obtained by fractional distillation in corresponding temperature ranges. This indicates that the thermal catalytic cracking process used in this study, with 20% (w/w) sodium carbonate as the catalyst, resulted in distilled fractions with a higher bio-hydrocarbon content than traditional fractional distillation methods. Specifically, the bio-hydrocarbon content in green gasoline (Range 2) is significantly higher than that obtained by Wisniewski et al. [50] from reactive distillation in the temperature range of <200 °C. However, it shows a similar bio-hydrocarbon content to the results of Zhao et al. [17], who obtained a distilled fraction with 91.97% bio-hydrocarbons using simple distillation in the temperature range of <200 °C. This indicates that this study’s thermal catalytic cracking process can yield high bio-hydrocarbon content comparable to or even better than traditional distillation methods.

Furthermore, the green kerosene and green diesel fractions obtained in the present study demonstrate similar bio-hydrocarbon contents to those reported by da Silva Almeida et al. [51] and da Silva Almeida et al. [14] for kerosene and light diesel fractions, respectively, obtained by fractional distillation at 175–235 °C and 235–305 °C. Overall, the comparison in Table 13 supports the efficacy of the thermal catalytic cracking process with sodium carbonate catalyst in producing biofuels with high bio-hydrocarbon content, making them potentially valuable alternatives to traditional fossil fuels. These results contribute to the growing body of research on sustainable biofuels and their potential to meet energy demands with reduced environmental impact.

4. Conclusions

The use of 10% (w/w) of reused sodium carbonate without a previous regeneration step in the thermal catalytic cracking process of crude palm oil reduces the yield and the quality (regarding the physical-chemical properties) of the OLP when compared with the same parameters for OLP from cracking that used 10% (w/w) of fresh sodium carbonate. Despite this, the reuse of sodium carbonate without any regeneration step proved to be viable, since, despite the reduction in its catalytic activity, it remains with the crystalline phase that characterizes it, which is the natrite phase. It managed to break the triglyceride molecules into oxygenated compounds (carboxylic acids, fatty alcohols, ketones) and, mainly, bio-hydrocarbons such as alkanes and alkenes, promoting higher yield and better quality than those found for commercial catalysts. This even indicates that sodium carbonate can be reused more than once in the cracking process.

The reaction time significantly influenced the values of the physical-chemical properties and chemical composition of the OLPs during cracking crude palm oil. In such a way, as the reaction time increased from 0 to 60 min, these values of the physical-chemical properties reduced slightly for the three conditions employed: 10% (w/w) of fresh sodium carbonate as the catalyst, 20% (w/w) of fresh sodium carbonate, and 10% (w/w) of reused sodium carbonate as the catalyst. The reaction time of 30 min is the minimum cracking time for the three conditions employed that promote the production of an organic liquid product with physical-chemical properties and chemical composition close to or within limits established for petroleum diesel by national and international regulatory agencies.

The fractional distillation at 1 atm of the OLP improves the values of the physical-chemical properties of the biofuels (green gasoline/Range 1, green gasoline/Range 2, green aviation kerosene and green diesel), promoting a similarity of the experimental distillation curves with standard distillation curves and allowing the majority of the values of the physical-chemical properties of the distilled fractions to be within limits established by the ANP. Besides, a single fractionation of OLP generates biofuels (green gasoline, green aviation kerosene, and green diesel) with higher bio-hydrocarbon contents than oxygenated compounds and which are composed of bio-hydrocarbons characterizing the respective petroleum derivatives. Therefore, biofuels have a promising potential to be used as new products in renewable energy, allowing the current consumer market to purchase products that can partially or entirely replace petroleum products, as do bioethanol and biodiesel.